Mirror Class rules allow variation in the spinnaker sheet system. As far as I can see, going right back to the mid-70's, systems using continuous sheet and guy have been the norm. There have been variations, for example the system with a "tensioner" descibed in Roy Partridge's book "Sailing The Mirror".
At the time of writing the universally popular system is continuous sheets with turning blocks near the shrouds. In time other systems will be added for historical interest and completeness.